A conventional thermal transfer printer is configured such that thermal heads, serving as heating elements, are arranged one-dimensionally (main scanning direction), and recording paper or film is successively fed in a printing direction (sub scanning direction).
The back of an ink ribbon superposed on the recording paper or film is heated by means of the thermal heads in order to transfer the ink (color material) of the ink ribbon onto the recording paper or film by sublimating or melting the ink.
Each of the thermal heads is a composed of a plurality of heat-generating resistors formed on a substrate in a row. The thermal transfer printer includes a plurality of ink ribbons corresponding to a plurality of colors. The inks of the ink ribbons of the plurality of colors (e.g., yellow Y, magenta M, cyan C, and black K) are transferred in a superposed state onto the recording paper at the same position, to thereby perform color printing. For example, the plurality of ink ribbons are disposed to be rotatable, and an ink ribbon to undergo thermal transfer is moved to the position of the thermal heads.
Each of the thermal heads can control the amount of applied heat stepwise. However, when it melts and transfers a color material, it is susceptible to the density of dots as well as to adjacent dots, and encounters difficulty in controlling gradation on a pixel-by-pixel basis. Therefore, the melting and transfer of the color material is controlled by use of two values; i.e., a value representing “to be transferred” and a value representing “not to be transferred.” In such a case, dots of a certain size are drawn, whereby a gradation is expressed through area modulation. For example, there are used a halftone generation method in which a rational tangent matrix is used, and a super-cell-scheme halftone generation method which is based on the former method and in which a plurality of matrices are used in order to virtually increase the number of gradation levels.
In a known method, an image signal is processed by means of a correction circuit for image-signal processing which includes a gradation correction table, the processed image signal and an image signal from an external apparatus are selectively output through switching, and the output image signal is subjected to gradation processing to be output. In another known method, input image data are spatially divided into matrices, and gradation conversion is performed on the basis of a growth-start gradation value of each pixel determined in accordance with the priority order sequentially determined from a pixel located at the center of each matrix toward pixels located at the outer edge of the matrix (see, for example, Patent Document 1 and Patent Document 2).    Patent Document 1: Japanese Patent Application Laid-Open (kokai) No. H11-177826    Patent Document 2: Japanese Patent Application No. 2005-096406